1. Technical Field
The present invention relates to a method for removing a formation of an oxide of titanium that is generated as a byproduct of a process that forms cobalt disilicide within an insulated-gate field effect transistor.
2. Related Art
Integrated circuits constructed on silicon substrates are limited in performance by the resistance of the source, drain, and gate of a field effect transistor (FET). This difficulty may be addressed by forming cobalt disilicide areas within FETs, since the cobalt disilicide areas have low electrical resistance. For example, cobalt disilicide within the source and drain of an FET lowers the total resistance for current flow from a contact, through the FET source, into the FET channel, and out of the FET drain.
In the formation of cobalt disilicide, it is well known to utilize a layer of cobalt as a source for the cobalt disilicide and to use a titanium nitride (TiN) capping layer to protect the cobalt from oxidizing during a subsequent annealing step. After a first annealing step, this sacrificial TiN capping layer is chemically removed by a selective etch with a solution such as one comprising hydrogen peroxide. Discrete portions of the TiN cap are not always removed by this process, however, and a residual configuration, or xe2x80x9cstringer,xe2x80x9d of one or more oxides of titanium, such as titanium dioxide, may remain after the cobalt disilicide is formed in a second annealing, step. Unfortunately, the stringer of a titanium oxide is electrically conductive and may cause electrical shorting of adjacent structures. For example, the stringer may cause a short between the gate and the drain of an FET, between the source of a first FET and the drain of a second FET, or between the drain of an FET and external circuitry. The prior art does not disclose a method of removing a stringer of an oxide of titanium that is generated as described above.
The present invention provides a method of removing an oxide of titanium which is generated as a byproduct of a process that forms cobalt disilicide within a semiconductor device such as an FET.
The present invention provides an FET within a substrate, wherein the FET is a gate-insulated field effect transistor comprising a source, a drain, a gate, a gate insulator, and a channel. Typically, the substrate is first precleaned with a suitable reagent such as hydrofluoric acid (HF). This precleaning removes a film of silicon dioxide (SiO2) that became deposited on a surface of the layer of silicon as a consequence of prior processing or of prior exposure of the substrate to air at room temperature. Next, a layer of cobalt is formed on a top surface of the substrate by use of a sputtering process such as with argon gas in a low-pressure chamber. The top surface of the substrate comprises a portion of the top surface of each of the source, drain, gate, and insulating structures. Next, a layer of titanium nitride is formed on the layer of cobalt by use of a second sputtering process such as with argon gas and nitrogen gas in a low-pressure chamber. Then a first annealing of the substrate causes portions of the layer of cobalt to react with the source, drain, and gate to transform a top portion of each of the source, drain, and gate into a silicide zone comprising a greater amount of cobalt silicide (CoSi) and a lesser amount of cobalt disilicide (CoSi2). Unreacted cobalt remains after the preceding annealing step, particularly on top of the isolating structures. The layer of titanium nitride and the unreacted cobalt are removed by a first cleaning with a reagent such as one comprising hydrogen peroxide and sulfuric acid. Impurities comprising titanium, cobalt, silicon, oxygen, and/or nitrogen may be present on the substrate after the first cleaning and a second cleaning is performed to remove the impurities. The first and second cleanings in combination may not successfully remove all impurities and impurities comprising titanium may be present on the substrate. Next, a second annealing process transforms cobalt monosilicide to cobalt disilicide in the silicide zone, thereby forming the desired cobalt disilicide within the FET. Nonetheless, a stringer of an oxide of titanium may be present on one or more of the cobalt disilicide areas of the silicide zone following the second annealing, and all such stringers should be removed to prevent shorting of adjacent electrical structures of, within, or coupled to, the FET. The final step removes the stringers by applying a reagent to the substrate at a suitable temperature, and for a period of time, wherein the reagent does not chemically react with the cobalt disilicide.
Use of an FET in the preceding method is illustrative. The preceding process steps may be applied to any semiconductor structure to form cobalt disilicide volumes that are free of stringers of an oxide of titanium.
Thus, the invention has the advantage of forming cobalt disilicide by a process that does not leave stringers of one or more oxides of titanium.